RF sputtering apparatus may include an evacuatable chamber, commonly referred to as a vacuum chamber or plasma reactor, that includes at least two electrodes, between which a plasma can be formed. At least one of the electrodes provides the material to be sputtered and at least one other electrode provides a counter electrode. In RF sputtering, a high frequency voltage is applied between the two electrodes, which continuously alternates in respect of polarity.
It is observed that the electrode with the smaller electrode surface displays a preferential sputtering effect. Therefore, a smaller electrode is typically used as the electrode comprising material to be sputtered and a larger electrode is used as the counter electrode that is typically connected to ground.
The sputtering-off effect is however not completely limited to the smaller electrode area; the extent to which the larger electrode is affected by sputtering depends on the difference between the plasma potential and the potential of the larger electrode. If this difference exceeds the sputtering threshold, the larger electrode will also be sputtered. This is undesirable if the larger electrode comprises one or more elements which are not desired to be sputtered from the larger electrode and deposited onto the substrate.
In order to avoid this effect of sputtering from the larger electrode, the enclosure of the RF sputter arrangement (e.g. a vacuum chamber) may be used as the larger counter electrode. The ratio between smaller electrode area and larger counter electrode area can be 1:10 or higher which is observed to reduce sputtering from the larger electrode.
This design rule of 1:10 however has limits: Certain applications, e.g. the treatment of wafers with 30 cm diameter normally require a sputter target of 40 cm. The 1:10 rule would then result in a counter electrode area of more than 1 m2, which is difficult to arrange in a vacuum sputter chamber.
GB 2 191 787 discloses an arrangement in which a magnetic field is applied at the counter electrode in order to enhance the effect of the counter electrode and reduce sputtering of the counter electrode. The ratio of the counter, electrode area to the target electrode area can be reduced by use of the magnetic field. This enables a counter electrode of a smaller area to be used whilst still avoiding undesirable sputtering from the counter electrode.
However, this approach requires additional magnets which complicates the design and manufacture of the sputtering apparatus. Therefore, further arrangements for RF sputtering are desirable which also reduce the likelihood of sputtering of the counter electrode.